1. Field of the Invention
The invention relates to full division duplex (FDD) systems, and in particular, to methods and apparatuses for blocker filtering in a WCDMA system in which a transmitter and a receiver are simultaneously enabled.
2. Description of the Related Art
Referring to FIG. 1a, signal tones distributed on a frequency axis are shown. In wireless cellular systems, blockers originate from either a self transmitter, such as a WCDMA, or from a users' transmitter such as a GSM. As shown in FIG. 1a, a large blocker TX is always present when a weak desired signal RX is received. The blocker is typically a self transmitter in full division duplex (FDD) systems, such as a WCDMA, since the transmitter and receiver are simultaneously enabled. The blocker most often degrades the sensitivity of the receiver in three ways. First, the blocker could saturate the receiver, and secondly, the blocker could inter-modulate with strong jammers to generate in-band cross-modulation distortion (XMD). Thirdly, the blocker could generate low frequency 2nd-order inter-modulation distortion (IMD2) at baseband along with the desired received signal. Therefore, due to the blocking effects, integrated receiver design is stringently hindered.
Referring to FIG. 1b, a conventional FDD system 100 utilizing a surface acoustic wave (SAW) filter 105 is shown. In the FDD system 100, a duplexer 104 is deployed on the input of the receiver 110 to reject the out-of-band blockers by 45-55 dB on average, and also to scale down the transmitter 120 power amplifier (PA) noise floor to at least 10 dB below the thermal noise (kTB in 3.84 MHz bandwidth). To further lower the distortions generated by the blockers such as the signal tone TX in FIG. 1a, an off-chip RF SAW filter 105 with typically 20-25 dB blocker rejection is deployed in the receiver 110, coupled between a low noise amplifier (LNA) 112 and a down converter 116. The use of the SAW filter 105, however, introduces several drawbacks. First, they have 2-3 dB insertion loss at the desired receiver band. Secondly, the output of LNA 112 needs to be matched to the input impedance of the SAW filter 105, i.e. 50 ohm. To compensate for lower load resistance, the LNA 112 consumes more bias current to retain the high gain. Thirdly, the output of the SAW filter 105 needs to be matched to the input impedance of the proceeded stage, which is typically a down converter 116 whereby the mixer noise is further lowered. Finally the SAW filter 105 is an off-chip component that degrades the integration level of transceivers and increases costs. It is therefore necessary to develop an enhanced approach to substitute for the SAW filter 105.